PBI modification and cross-linking methods

ABSTRACT

The present disclosure provides methods for modifying and cross-linking polybenzimidazoles, PBI. In one embodiment, the polybenzimidazole reacts with a compound, which has a halogen and a double bond functionality and which comprises a halogen and an organic group to form modified polymers by means of a nucleophilic substitution of the amine proton of the benzimidazole functionality in a solution, and a functional group is connected via each resulting free double bond and/or the polymers that are thus modified are cross-linked.

PRIORITY CLAIM

The present application is a divisional application of U.S. patentapplication Ser. No. 14/979,039, filed Dec. 22, 2015 now U.S. Pat. No.9,873,768, which is a divisional application of U.S. patent applicationSer. No. 13/821,390, filed May 28, 2013 now U.S. Pat. No. 9,221,951 (371(c) date), which is a national stage entry of International ApplicationNo. PCT/DE2011/001690, filed on Sep. 7, 2011, which claims priority toGerman Patent Application Number DE 10 2010 044 579.7, filed on Sep. 7,2010. The entire disclosure of each of these applications isincorporated by reference herein.

SUMMARY

The present disclosure provides methods for the modification andcross-linking of polybenzimidazole (“PBI”).

PBI fibers, a product of space exploration in the 1980s, originallyserved as the upper material of fire protective clothing.

Because of its extraordinary thermal and chemical resistance, PBI hasnow found its way into the production of fuel cells as a membranematerial and is used especially as a material for high-temperaturemembranes in polymer electrolyte fuel cells (“PEFC”). PBI often servesas a matrix for proton-conducting phosphoric acid because PBI withstandsthe high temperatures of such fuel cells without problem but is itself avery poor proton conductor, and therefore regularly needs correspondingdoping.

Such doping has the advantage that by the choice of a suitable dopant,membranes can be produced for fuel cells for both acidic and alkalinefuels, for example with KOH as a dopant in the case of alkaline fuels.

However, a disadvantage of such doping is the migration of the dopantduring operation of the fuel cell, so that the initial high protonconductivity then decreases significantly over the lifetime of the fuelcell.

Another disadvantage is the low mechanical stability of highly doped PBImembranes. This can be encountered, for example, in the case ofcross-linking of polymers with difunctional halogen compounds accordingto U.S. Patent Publication No. 2004/0261616 or difunctional epoxides andisocyanates according to published German Patent Application No. DE 10110 752 A1. However, in the case of the methods described there, thecross-linking reaction and the competing doping process both take placebecause of the imidazole functionality, especially in the case of theamine proton.

Against this technical background, the present disclosure providesmethods for preparing a modified PBI polymer that is easy tomanufacture, and, in particular when used as a starting material for themembrane, can be largely freely functionalized and/or cross-linked.

DETAILED DESCRIPTION

This technical problem is solved by the procedure as disclosed herein.

In one embodiment, a PBI with the structure

is reacted in a solution, with a compound of a halogen and a double bondfunctionality of the type

where X is a halogen and R an organic group, for example an alkylhalide, in particular 3-bromopropene, which by a nucleophilicsubstitution of the amine proton of the benzimidazole functionalityenables the modified polymers

to be obtained. The free double bonds are now available forcross-linking or functionalization of the thus modified polymer in asimple manner.

The modified polymers in the form of precipitated powder or granules canbe mixed later with a cross-linking agent also in powder form undersuitable reaction conditions in order to form a molded part.

If a molded part such as a membrane or a film is produced from thesolution, then the modified polymers, in particular allyl-functionalizedpolymers, can be cross-linked directly or indirectly to one another withor without an initiator, whereby a non-soluble molded part is obtained.

The cross-linking between two modified polymers can be obtainedindirectly via a cross-linking molecule having at least two doublebonds. After the successful reaction of the original PBI, a compoundhaving a halogen and a double bond functionality is added to thesolution, which then has the modified polymers or, after production of amolded part, this may be introduced into a solution together with acomponent not dissolving the molded part along with the cross-linkingagent, and the cross-linking agent diffuses into the molded part.Cross-linking is then obtained again through associated heat treatment.

A particularly stable cross-linking is the direct crosslinking of twomodified polymers via two double bonds, which is described below in theexplanation of an embodiment.

In one embodiment, a polymer solution with polybenzimidazole having thestructure

is obtained by the addition of LiCl to improve the solubility and by theaddition of a catalyst, preferably a bicyclic tertiary amine, such astriethylenediamine or 1,4-diazabicyclo[2.2.2]octane or TEDA or DABCO indimethylacetamide, DMAc, is used as the solvent.

4n 3-bromopropene (allyl bromide) is added as a compound having ahalogen and a double bond functionality:

Following a reaction time of about 8 to 24 hours, 4n HBr can be desorbedby heating the solution to about 40° C., and a modified polymer isobtained having the structure

Films can be drawn from the solution and the LiCl washed out.

The subsequent cross-linking is effected in an oven under the influenceof temperature to form:

where C-L stands for cross-linking and can represent one of theabove-mentioned bonding functions.

Surprisingly, the failure temperature of the modified cross-linkedpolymers at around 528° C. when tested by thermogravimetric analysis isonly slightly lower than that of the original polymers at around 536°C., but this was expected due to the linking of an aliphatic chain.

On the other hand, the behavior of the modified cross-linked polymerswhen subjected to dynamic mechanical analysis, denotes a significantlyhigher modulus of elasticity of the modified cross-linked polymers athigh temperatures, which indicates very good cross-linking.

Accordingly, in one embodiment, cross-linking can be provided thatconnects a functional group with at least two double bonds to a doublebond of a modified polymer.

Thus from the point of view of acidic membranes for fuel cells, it isconsidered in particular that the functional group would have a highproton conductivity, such as vinylphosphonic acid, or1-allyl-3-methylimidazolium chloride.

Thus one obtains an acidic PBI when a stoichiometric amount ofvinylphosphonic acid to the allyl units and an initiator such astert-butyl perbenzoate is added to a 3% aqueous solution of anallyl-functionalized PBI in DMAc as described above.

The reaction solution is heated under nitrogen at 140° C. for 4 hours toreflux.

The functional group may also be an amine group, through which, inparticular, the existing alkaline properties of the PBI can be furtheremphasized. This may be beneficial in the production ofH₂/CO₂-selective, alkaline then anion-conducting gas-separationmembranes.

For such membranes, it may also be advantageous when the functionalgroup is based on an ionic liquid, for example, connected to theallyl-bonding imidazolium. In the case of membranes based on known ionicliquids, it is known that loss of conduction may occur due to migrationof the ionic liquid. By using 1-allyl-3-methylimidazolium chloride, onecan connect the ionic liquid covalently to the modified polymer, andthus prevent the migration.

In another embodiment, the functional group decreases the degree ofcrystallization of the polybenzimdazole, for example by the connectionof a bulky group such as allylbenzene or allyl p-toluol sulfate.

In addition, the formation of copolymers is not a problem when a monomerhaving a double bond is connected to the double bond of a modifiedpolymer, which can, for example, take place by means of a radicalpolymerization.

As an example of functionalization and cross-linking of the modifiedpolymers, reference is also made to the possibility of producing a filmor a membrane made from an allyl-functionalized PBI and then soaking itin an appropriate solution, for example, vinylphosphonic acid, ifnecessary with the addition of a cross-linking agent, in order to obtaina reaction between the allyl function and the vinyl phosphonic acid inan oven and achieve the cross-linking.

Another example of functionalization and linking of the modified polymeris the addition of triallyl isocyanurate, tradename TAIC, known as aco-activator for peroxide cross-linking, which enables a variety ofthree double bond cross-linking possibilities. Furthermore, triallylisocyanurate, as a polyfunctional allylic monomer, can itself polymerizeor effect a connection of a functional group to one of the double bonds.

The invention claimed is:
 1. A method of cross-linking apolybenzimidazole (PBI) having the structure

wherein the polybenzimidazole reacts in a solution with a compoundhaving a halogen and a double bond functionality of the type

where X is a halogen and R an organic group through a nucleophilicsubstitution of the amine protons of the benzimidazole functionality toobtain the modified polymers

where the resulting free double bonds are connected in each case to afunctional group and/or crosslinking of the modified polymers occurs. 2.The method of claim 1, wherein the modified polymers are cross-linked toform

wherein C-L is a crosslinker, and wherein the polymer further includesan amine functional group.
 3. The method of claim 2, wherein thecrosslinker comprises one or more of:


4. The method of claim 1, wherein the cross-linking occurs between twomodified polymers.
 5. The method of claim 4, wherein the cross-linkingbetween two modified polymers occurs by means of a cross-linkingmolecule having at least two double bonds.
 6. The method of claim 4,wherein the cross-linking of two modified polymers includes two doublebonds.
 7. The method of claim 1, wherein at least one functional grouphaving a double bond is connected to the modified polymer.